The greatest force which a vehicle wheel can exert parallel to the surface over which it moves depends on the nature of the wheel and of the surface, commonly expressed as a coefficient of friction, and on the force exerted by the wheel in the direction normal to the surface. Distribution of tractive force among driving wheels so as to equalize, to the extend possible, the required coefficient of friction, improves vehicle safety.
Performance of vehicles which carry loads of different weight is improved by means which allow adjustment of relative wheel traction in approximate proportion to static wheel loading. Performance of vehicles required to maneouver rapidly is improved by automatic control of relative wheel traction.